Electric reduction furnaces of round construction have three electrodes which are arranged on a pitch circle and form the tips of a substantially equilateral triangle. In a cylindrically shaped furnace considerable dead spaces are present between every two adjacent electrodes and the wall of the furnace. Due to this arrangement of the electrodes, the electric reduction furnaces include, at the same time, zones on the furnace wall which are subjected, corresponding to the distance of the electrodes, to higher thermal stresses during the operation of the furnace.
With electric reduction furnaces of rectangular construction, the electrodes are arranged in a row. Furnaces of this type also have zones of high thermal stress, as well as dead spaces.
In electric reduction furnaces, the electrodes extend either into the melt or into the burden to a short distance above the melt. In the latter case, a so-called "dome formation" takes place during the melting process in the region between the tip of the electrode and the melt when the burden is charged separately, i.e. a dome of coarsely granular carbon which has separated out of the burden. A part of the electric current flows into the melt via the wall of the dome, which is electrically conductive.
With treated burden, such as pellets or briquettes, the separation of the burden does not take place to this extent, so that in this case, instead of the dome of carbon, electrically conductive banks of pellets or briquettes are produced between the electrodes.
Since all three electrodes are connected to the melt via domes or banks, a star-shaped connection from the standpoint of a wiring diagram, is present here.
The domes as well as the banks of the individual electrodes are, however, at the same time connected to each other. There is thus also a flow of current between the individual electrodes as a function of the corresponding phase of current at the time. From the standpoint of a wiring diagram, a delta connection is present here.
The same situation is present from the standpoint of a wiring diagram when the electrodes dip into a molten bath. The volumes of slag below the electrodes towards the furnace bath and between the electrodes form the current paths and thus bring about the heating mechanism.
Furnaces having the above-mentioned heating mechanism are employed in numerous steel plants.
An electric arc furnace is known from German Patent Application No. AS 1,050,464, in which the shape of the vessel of the furnace is adapted to the active regions of the individual electrodes. This embodiment has the disadvantage of the complicated shape of the mechanical part and the refractory part of the furnace.
From German Pat. No. 973,715 a three-phase current reduction furnace is known which has six electrodes, five of which are arranged in a circle and one in the center of the circle. This arrangement has the disadvantage that, on the one hand, two additional electrodes are used at a slight distance from the wall of the furnace, the electrodes producing a high thermal stressing of the furnace lining and, at the same time, causing a chemical decomposition of the refractory material as a result of the flow of electric current. This decomposition of the refractory material is known, and is indicated by the manufacturers in kg per ampere-hour for the refractory material in question.